Voice coil motor (VCM) is the mainstream technology to linearly drive a lens to achieve autofocus function of a camera module in a mobile phone. Many different VCM designs have been invented to provide such lens driving capability. Such VCM designs or lens driving apparatus comprise a pair of leaf springs, magnets, coil, and yoke. However, VCMs disclosed by the mentioned patents and patent application do not comprise a position sensor or a position encoder, which can measure the lens position and feedback a signal or an electronic signal for lens position control. In fact, those disclosed VCMs control the lens position by precisely controlling the operating current of the VCM to achieve a mechanical force balance of the electromagnetic force and elastic force of springs. Such balance is done naturally without knowing the lens position. Therefore, the actual lens position will be varying for various camera postures for the same operating current. Furthermore, due to the spring-mass structure, such VCM will always create a lens position oscillation when the lens moves. Such position oscillation will cause blurring of images during focusing.
To solve the above problem, a position sensor or encoder is introduced into VCM. The output signal of the position encoder is sent to a position control driver integrated circuit (IC), which in turn drives the lens to the expected position. Such VCM includes a Hall element (or Hall sensor) used for lens position measurement. Such VCM operating mechanism is described as follows. VCMs are connected to an electronic circuit (usually an IC) which is used to receive the lens position signal from the Hall sensor. The position signal is analyzed by the electronic circuit and is used to find out the actual physical position. The electronic circuit then sends out a driving signal to drive the lens to move. During the lens movement, the Hall element keeps sending back the position signal, which is continuously analyzed to get the real physical position information. Based on the position information, the electronic circuit will determine to stop sending out the driving signal when the lens arrives the expected position. Otherwise, the circuit will continuously send out the driving signal to drive the lens to move. Such control loop is called close-loop control. Recently, such close-loop control VCM is getting more and more market traction due to its fast response, lower power consumption, and lower cost.
The above description of the background is provided to aid in understanding a lens driving apparatus, but is not admitted to describe or constitute pertinent prior art to the lens driving apparatus, or consider the cited documents as material to the patentability of the claims in the present application.